The present invention relates to plastic injection molding systems and more particularly to gas-assisted plastic injection molding systems and processes.
There are numerous known systems for plastic injection molding. In conventional plastic injection molding systems, plastic pellets are melted in an injection molding machine and advanced by a screw ram into a mold cavity, typically through one or more sprue bushings, a manifold, and/or a hot runner system. The mold cavity is formed between two mold halves (a core member and a cavity member). The two halves of the mold are clamped, typically under high pressure, and the plastic is injected into the mold cavity, again under significant pressure in most instances. The molten plastic material in the cavity is allowed to cool and harden in the cavity, typically by a cooling system which circulates a cooling fluid through one or more of the mold members. When the part is sufficiently hardened, the mold is opened and the part is removed, typically by one or more ejector pins.
Some of the known systems utilize a gas in the injection molding process and are commonly known as “gas-assisted injection molding” systems. In these systems, the gas is injected into the molten plastic material through the plastic injection nozzle itself, or through one or more pin mechanisms strategically positioned in the mold, sprue bushings, manifold, or hot runner systems. It is also possible to inject the gas directly into the molten plastic in the barrel of the injection molding machine. The gas, which typically is an inert gas such as nitrogen, is injected under pressure and forms one or more hollow cavities or channels in the molded part. The pressurized gas applies an outward pressure, forcing the plastic against the mold surfaces while the article solidifies. This helps provide a better surface on the molded article, and also reduces or eliminates sink marks and other surface defects. The use of pressurized gas also reduces cycle time as the gas is introduced and/or migrates to the more fluent inner volume of the plastic and replaces plastic in those parts which would otherwise required an extended cooling cycle. The benefits of gas-assisted injection molding processes are well known and include the cost savings through the use of less plastic material, producing parts which are lighter in weight, and producing parts which have better surface definitions and finishes.
In the plastic injection molding art, the usual challenges facing the product designer include designing an article having the requisite strength for the product application and satisfactory surface finish as well as avoiding excessive weight, surface distortions, and increased cycle time. For flat or thin products, it is typical to include one or more rib members in the design to provide relative strength and structure for a molded article. The rib members are typically thicker than the molded article, and the rib members, along with any other desired thicker portions, increase the weight material usage, and cycle time of the plastic article. These members and/or portions also often induce sink marks and other surface defects due to thermal gradients in the area of the thickened portions.
Where the rib members or other portions of the article in which the gas is being introduced are elongated, it is often difficult to provide a satisfactory molded article for additional reasons. For example, if the pressure of the gas is too great as it enters the mold cavity, there is a risk that it may rupture or blow out the plastic within the mold cavity, i.e., the gas is not contained within the molten plastic material. Also, it is often difficult to have the gas migrate along the full length of an elongated, thicker plastic section, thus creating a product which has an uneven thickness and cooling cycle. This can lead to undesirable stresses and/or deformation of the molded part.
Some gas-assisted plastic injection molding systems and processes have been developed in order to overcome some of the above-mentioned problems. In some of these processes, secondary reservoirs or cavities are provided adjacent the molded part or elongated rib members in order to collect and contain the plastic material which is forced out of the article or rib member by the pressurized gas. Although many of these gas-assisted injection molding systems and processes operate satisfactorily and have produced commercially acceptable plastic injection molding parts and components, these processes use excess plastic material and require excess processing steps. There is a need for improved systems and processes in the gas-assisted injection molding field which do not utilize secondary cavities, particularly since such processes use excess plastic which often cannot be reused, or which requires capturing and regrinding.